This is the principle antibody found in blood and body fluids. Around 75% of the antibody circulating in the blood is IgG, and it is this isotype which provides the majority of the antibody-mediated protection against infection.
IgG is rarely produced during an initial response to a given pathogen: this antibody is not produced until around one month initial B cell activation. The principle function of IgG is to bind to pathogens via an antigen-specific receptor, and form antigen-antibody complexes. The formation of these complexes targets the pathogen for destruction via other immune cells.
Note – IgA and IgE have similar functions. All three antibodies have a receptor called the Fc receptor, which interacts with other immune cells such as phagocytes and granulocytes. When this interaction occurs, the cells are triggered to destroy the antibody-antigen complexes, thus also destroying pathogens or their toxic secretions.
IgG also activates the complement system, a cascade of reactions between a set of proteins which results in the formation of a molecule capable of destroying bacterial cells.
In addition, IgG is the only antibody which is able to cross the placental barrier, thus providing passive immunity to a developing fetus.